Ion sensing device

ABSTRACT

Ion sensing device includes a field-effect transistor including a bottom gate and a top gate, a reference electrode, and a driver circuit configured to measure concentration of ions in a sample solution into which the reference electrode and the top gate are immersed. The driver circuit includes a constant current source configured to supply a drain of the field-effect transistor with a constant current, and a voltage follower configured to receive a potential of the drain. The driver circuit is configured to supply the reference electrode with a constant reference potential, apply a constant voltage across an output of the voltage follower and the bottom gate, and output an output potential of the voltage follower.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. application Ser. No.17/099,976 filed Nov. 17, 2020, which claims benefit of non-provisionalapplication claims priority under 35 U.S.C. § 119(a) on PatentApplication No. 2019-236925 filed in Japan on Dec. 26, 2019, the entirecontent of which is hereby incorporated by reference.

BACKGROUND

This disclosure relates to an ion sensing device.

Ion-sensitive field-effect transistors (ISFETs) are used in the fieldsof pH measurement and biosensing. An ISFET does not have a gateelectrode of a field-effect transistor. In measurement of an ionconcentration with an ISFET, a reference electrode is immersed in thesample solution and the gate insulating film of the field-effecttransistor is exposed to the sample solution. As a result, an electricaldouble layer is produced at the interface between the sample solutionand the insulating film of the ISFET and the voltage of the electricaldouble layer changes the potential at the interface of the channel ofthe ISFET.

A method of evaluating an ion concentration with an ISFET can besummarized as follows:

-   -   (1) Immerse the gate insulating film in the sample solution        which is also in contact with the reference electrode;    -   (2) Apply a constant voltage across the source and the drain of        the ISFET; and    -   (3) Change the source potential with respect to the reference        potential so that the drain current will be constant.

The voltage across the reference electrode and the gate changes inaccordance with the ion concentration of the sample solution. The sourcepotential changes in accordance with the change of the voltage acrossthe reference electrode and the gate. The source potential is the outputof the ISFET; the ion concentration of the sample solution can beobtained from the source potential. The theoretical maximum sensitivityof the ISFET is 58.16 mV/mol (T=293.15K), according to the Nernstequation.

Techniques to improve the sensitivity of the ISFET are disclosed in US2017/0082570 A and US 2015/0276663 A, for example. The driver circuitdisclosed in US 2017/0082570A converts the drain current of thetransistor to voltage with a resistor, detects the voltage, and controlsthe voltage across the bottom gate and the source with a microprocessorso that the drain current will be constant. As a result, the voltageacross the bottom gate and the source is obtained as the variation inthe top gate voltage in accordance with the ion concentration. Thedriver circuit disclosed in US 2015/0276663 A is composed of only analogcircuit elements and does not include a microprocessor.

SUMMARY

An aspect of this disclosure is an ion sensing device including: afield-effect transistor including a bottom gate and a top gate; areference electrode; and a driver circuit configured to measureconcentration of ions in a sample solution into which the referenceelectrode and the top gate are immersed. The driver circuit includes: aconstant current source configured to supply a drain of the field-effecttransistor with a constant current; and a voltage follower configured toreceive a potential of the drain. The driver circuit is configured to:supply the reference electrode with a constant reference potential;apply a constant voltage across an output of the voltage follower andthe bottom gate; and output an output potential of the voltage follower.

Another aspect of this disclosure is a method of driving a referenceelectrode and a field-effect transistor including a bottom gate and atop gate in order to measure concentration of ions in a sample solution.The method includes: supplying the drain of the field-effect transistorwith a constant current; inputting a potential of the drain to a voltagefollower; supplying the reference electrode with a constant referencepotential; applying a constant voltage across an output of the voltagefollower and the bottom gate; and outputting an output potential of thevoltage follower. The method is performed in a state where the referenceelectrode and the top gate are immersed in the sample solution.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram schematically illustrating aconfiguration example of a sensor transistor included in an ion sensingdevice in an embodiment of this disclosure;

FIG. 2 illustrates a configuration example of a driver circuit includedin a controller;

FIG. 3 illustrates a configuration example of a sensor transistor forselectively measuring the concentration of a specific kind of ions;

FIG. 4 schematically illustrates a configuration example of a sensortransistor array included in an ion sensing device; and

FIG. 5 illustrates a configuration example of an array driver circuitfor driving the sensor transistors in FIG. 4 .

EMBODIMENTS

Hereinafter, embodiments of this disclosure will be described withreference to the accompanying drawings. The embodiments are merelyexamples to implement the idea of this disclosure and not to limit thetechnical scope of this disclosure. Elements common to the drawings aredenoted by the same reference signs and each element in the drawings maybe exaggerated in size or shape for clear understanding of thedescription.

Hereinafter, a configuration example of an ion sensing device isdescribed. The ion sensing device includes a reference electrode to beimmersed in the sample solution, a sensor transistor, and a drivercircuit for driving these. The driver circuit supplies a constantreference potential to the reference electrode. The driver circuitsupplies a constant current to the drain of the sensor transistor,inputs the potential of the drain of the sensor transistor to a voltagefollower, and applies a constant voltage across the output of thevoltage follower and the bottom gate. The output of the voltage followeris output as the value representing the ion concentration of the samplesolution. This configuration allows the potential of the referenceelectrode to be fixed and further, increases the accuracy in the outputof the driver circuit.

FIG. 1 is a cross-sectional diagram schematically illustrating aconfiguration example of the sensor transistor included in an ionsensing device in an embodiment of this disclosure. In addition to thesensor transistor 10 illustrated in FIG. 1 , the ion sensing deviceincludes a controller (not shown) for measuring an ion concentrationwith the sensor transistor 10. The controller includes a driver circuitfor driving the sensor transistor 10. The details of the driver circuitwill be described later.

The configuration example of the sensor transistor 10 in FIG. 1 is athin film transistor. The sensor transistor 10 includes a bottom gateelectrode 103 provided on an insulating substrate 101. The insulatingsubstrate 101 can be made of glass or resin. The bottom gate electrode103 can be made of an aluminum-based alloy.

A bottom gate insulating film 105 is provided to cover the bottom gateelectrode 103 on the insulating substrate 101. The bottom gateinsulating film 105 can be a silicon oxide film.

An island-like semiconductor film 107 is provided on the bottom gateinsulating film 105. The semiconductor film 107 can be made of an oxidesemiconductor. Examples of the oxide semiconductor include amorphousInGaZnO (a-InGaZnO) and microcrystal InGaZnO. Oxide semiconductors suchas a-InSnZnO, a-InGaZnSnO, and ZnO can also be used. Oxidesemiconductors can provide transistors with better saturationcharacteristics than the other thin-film semiconductor materials.

A source electrode 109 and a drain electrode 111 are provided in contactwith parts of the top face of the semiconductor film 107. The sourceelectrode 109 and the drain electrode 111 can be made of titanium ormolybdenum.

A top gate insulating film (ion-sensitive insulating film) 113 isprovided to cover and be in contact with the semiconductor film 107 andparts of the source electrode 109 and the drain electrode 111. The topgate insulating film 113 can be made of a high-dielectric material suchas aluminum oxide, tantalum oxide, or hafnium oxide.

These high-dielectric materials have relative permittivities higher thanthat of silicon oxide. Accordingly, the electrostatic capacitance perunit area of the top gate insulating film 113 is higher than theelectrostatic capacitance per unit area of the bottom gate insulatingfilm 105.

A protection film 115 is provided to cover the top gate insulating film113, the source electrode 109, and the drain electrode 111 but expose apart of the top gate insulating film 113. The part of the top gateinsulating film 113 over the region corresponding to the channel of thesemiconductor film 107 is exposed from the protection film 115. Theprotection film 115 is made of an insulative material such as epoxyresin.

The sensor transistor 10 having the above-described structure is put ina sample solution 15 together with a reference electrode 13 in such amanner that the top gate insulating film 113 exposed from the protectionfilm 115 is immersed in the sample solution 15. The controller drivesthe reference electrode 13 and the sensor transistor 10 and measures theion concentration of the sample solution 15 from the signal obtainedfrom the sensor transistor 10.

FIG. 1 is to illustrate an example of the sensor transistor 10; the ionsensing device in this embodiment can employ a field-effect transistorof any structure (or material). For example, the insulating substrate101 can be excluded and the bottom gate electrode 103 can be replacedwith a silicon substrate. The material of the semiconductor film 107 isnot limited to an oxide semiconductor; amorphous silicon, polysilicon,or crystalline silicon can be used.

As described above, the reference electrode 13 and the top gateinsulating film (ion-sensitive insulating film) 113 of the sensortransistor 10 are exposed to the sample solution 15. An electricaldouble layer is produced at the interface between the sample solution 15and the top gate insulating film 113 and the voltage of the electricaldouble layer changes the potential at the interface of the channel ofthe sensor transistor. The electrical double layer depends on theconcentration of ions in the sample solution 15. Accordingly, theconcentration of ions in the sample solution 15 can be measured byreading the signal from the sensor transistor 10.

Hereinafter, a method of measuring an ion concentration with thereference electrode 13 and the sensor transistor 10. FIG. 2 illustratesa configuration example of a driver circuit 20 included in thecontroller. The driver circuit 20 supplies the reference electrode 13with a reference potential. The reference potential in the configurationexample in FIG. 2 is the ground potential (GND). A voltage Vsig isgenerated across the reference electrode 13 and the top gate of thesensor transistor 10 in accordance with the ion concentration of thesample solution 15.

The driver circuit 20 includes a constant current source 201 connectedwith the drain of the sensor transistor 10 and a constant voltage source203 for supplying the constant current source 201 with a constantvoltage Vpw with respect to a reference potential. The driver circuit 20supplies the drain of the sensor transistor 10 with a constant currentIref. The drain current Id of the sensor transistor 10 is equal to theconstant current Iref.

The reference potential in the configuration example in FIG. 2 is theground potential (GND) and the constant current source 201 is maintainedat a constant potential with respect to the reference potential. Thesource of the sensor transistor 10 is maintained at a constant referencepotential. This reference potential in the configuration example in FIG.2 is the ground potential.

The driver circuit 20 further includes an operational amplifier 205. Theoperational amplifier 205 is configured as a voltage follower. Thepotential of a node between the constant current source 201 and thedrain of the sensor transistor 10 is input to the non-inverting input(+) of the operational amplifier 205. The output of the operationalamplifier 205 is connected with the inverting input (−) to applynegative feedback to the operational amplifier 205.

The driver circuit 20 further includes a constant voltage source 207between the output of the operational amplifier 205 and the bottom gateof the sensor transistor 10. The constant voltage source 207 suppliesthe bottom gate of the sensor transistor 10 with a potential differentfrom the output of the operational amplifier 205 by a constant voltageVbt. Although all reference potentials in the configuration example inFIG. 2 are the ground potential, they can be at different values as faras individual reference potentials are constant.

Operation of the driver circuit 20 is described. The voltage Vtgs acrossthe top gate and the source of the sensor transistor 10 takes a value inaccordance with the ion concentration of the sample solution 15.Accordingly, the ion concentration of the sample solution 15 can bemeasured by measuring the top gate voltage Vtgs with respect to thesource potential.

The voltage across the drain and the source of the sensor transistor 10(the drain potential with respect to the source potential) Vds and thevoltage across the back gate and the source of the sensor transistor 10(the back gate potential with respect to the source potential) Vbgs areexpressed as follows:

Vds=Vout  (1)

Vbgs=Vout−Vbt  (2)

where Vout represents the voltage across the output of the operationalamplifier 205 and the source of the sensor transistor 10 (the outputpotential of the operational amplifier 205 with respect to the sourcepotential).

Since negative feedback is applied to the operational amplifier 205, thepotential difference between the inverting input and the non-invertinginput is zero. Accordingly, the drain potential Vds of the sensortransistor 10 is equal to the output potential Vout of the operationalamplifier 205. Meanwhile, the constant voltage source 207 supplies theback gate of the sensor transistor 10 with a potential lower than theoutput potential of the operational amplifier 205 by a voltage Vbt.

The drain current Id of the sensor transistor 10 can be expressed by apredetermined function f:

Id=f(Vbgs−(ctg/cbg)Vtgs,Vds)  (3)

where ctg represents the electrostatic capacitance per unit area of thetop gate insulating film and cbg represents the electrostaticcapacitance per unit area of the bottom gate insulating film. When theelectrostatic capacitance per unit area of the top gate insulating filmis larger than the electrostatic capacitance per unit area of the bottomgate insulating film as described above, the sensor transistor 10 canhave high sensitivity.

The driver circuit 20 limits the drain current Id of the sensortransistor 10 to the constant current Iref with the constant currentsource 201. Although the voltage Vds across the drain and the source ofthe sensor transistor 10 could vary as described above, when thesaturation characteristic of the transistor is good and the transistoris operating with the saturation characteristic, the drain current Idhighly depends on the back gate potential Vbgs and the top gatepotential Vtgs, compared to the drain potential Vds. Accordingly, Vds inthe formula (3) can be ignored. Further, since Ids is kept constant bythe current source in the driver circuit 20, the following relation isestablished:

Vbgs−(Ctg/Cbg)Vtgs=Constant Value  (4)

As noted from the formula (2), the back gate potential Vbgs is expressedby a linear expression of the output potential Vout of the operationalamplifier 205. Accordingly, the output potential Vout of the operationalamplifier 205 can be expressed by a linear expression of the top gatepotential Vtgs. Since the top gate potential Vtgs takes a value inaccordance with the ion concentration of the sample solution 15, theoutput potential Vout of the operational amplifier 205 represents theion concentration of the sample solution 15. The controller candetermine the ion concentration from the output potential Vout of theoperational amplifier 205 in accordance with a predetermined function.The controller displays the measured ion concentration on a displaydevice.

In an example, the driver circuit 20 operates the sensor transistor 10in the saturation region. This operation lowers the dependency of thedrain current Id on the drain-source voltage Vds more. As a result, thetop gate potential Vtgs can be measured more accurately.

As described above, the driver circuit 20 does not include amicroprocessor but is composed of only analog circuit elements. For thisreason, no circuit element for analog-to-digital conversion isnecessary, unlike the configuration including a microprocessor.Therefore, the factors to degrade the measurement accuracy can beminimized. Further, the potential of the reference electrode 13 can befixed at a desirable reference potential; the potential of the samplesolution 15 does not change during the measurement. For this reason,concentrations of a plurality of kinds of ions in the sample solution 15can be measured concurrently with one reference electrode 13 and aplurality of sensor transistors.

Hereinafter, an example of an ion sensing device for selectivelymeasuring the concentration of ions of a specific substance in a samplesolution 15 is described. FIG. 3 illustrates a configuration example ofa sensor transistor 17 for selectively measuring the concentration of aspecific kind of ions. Differences from the sensor transistor 10illustrated in FIG. 1 are mainly described.

The sensor transistor 17 includes a probe film 117 on a top gateinsulating film 113 to cover the exposed part of the top gate insulatingfilm 113, in addition to the configuration of the sensor transistor 10illustrated in FIG. 1 . The probe film 117 can be an ion-selective filmthat interacts with a specific kind of ions (for example, absorbs theions) or an organic film (such as an enzyme film or an antibody film)that interacts with a specific substance (for example, absorbs thesubstance) and raises the concentration of a specific kind of ions.

For example, a specific enzyme film raises the concentration of aspecific kind of ions within the film by enzymic reaction to a specificsubstance. The sensor transistor 17 can obtain an electric signalproportional to the concentration of the specific substance.

As understood from the above, the sensor transistor 17 can be used toselectively measure an ion concentration in accordance with theconcentration of a specific substance (including ions) in a samplesolution 15 The configuration example illustrated in FIG. 1 can also beused to selectively measure the concentration of a specific kind of ionsby forming the top gate insulating film 113 of a specific material. Suchsensing devices can be used in various fields such as biotechnology andmedicine.

FIG. 4 schematically illustrates a configuration example of a sensortransistor array 19 included in an ion sensing device. The sensortransistor array 19 includes a plurality of sensor transistors arrayedon a substrate. FIG. 4 illustrates two sensor transistors 17A and 17B byway or example. The structures of the sensor transistors 17A and 17B arethe same as the structure of the sensor transistor 17 illustrated inFIG. 3 and therefore, reference signs of some elements are omitted inFIG. 4 .

The sensor transistors 17A and 17B include probe films 117A and 117B,respectively, that interact with different substances. The sensortransistors 17A and 17B enable measurement of concentrations ofdifferent substances in a sample solution 15. An array driver circuit inthis embodiment allows the reference electrode 13 to be used commonly tothe sensor transistors 17A and 17B.

FIG. 5 illustrates a configuration example of an array driver circuit 25for driving the sensor transistors 17A and 17B illustrated in FIG. 4 .The array driver circuit 25 includes transistor driver circuits for thesensor transistors 17A and 17B in one-to-one correspondence. Eachtransistor driver circuit has the same configuration as the drivercircuit described with reference to FIG. 2 .

Specifically, FIG. 5 shows that a voltage VsigA is generated across thereference electrode 13 and the top gate of the sensor transistor 17A inaccordance with the concentration of ions of a specific substance in thesample solution 15. The array driver circuit 25 includes a constantcurrent source 201A connected with the drain of the sensor transistor17A and a constant voltage source 203 for supplying the constant currentsource 201A with a constant voltage Vpw with respect to a referencepotential. The array driver circuit 25 supplies the drain of the sensortransistor 17A with a constant current IrefA. The drain current IdA ofthe sensor transistor 17A is equal to the constant current IrefA. Theconstant current source 201A is maintained at a constant potential withrespect to the reference potential. The source of the sensor transistor17A is maintained at a constant reference potential.

The array driver circuit 25 further includes an operational amplifier205A. The operational amplifier 205A is configured as a voltagefollower. The potential of a node between the constant current source201A and the drain of the sensor transistor 17A is input to thenon-inverting input (+) of the operational amplifier 205A. The output ofthe operational amplifier 205A is connected with the inverting input (−)to apply negative feedback to the operational amplifier 205A.

The array driver circuit 25 further includes a constant voltage source207A between the output of the operational amplifier 205A and the bottomgate of the sensor transistor 17A. The constant voltage source 207Asupplies the bottom gate of the sensor transistor 17A with a potentialdifferent from the output of the operational amplifier 205A by aconstant voltage VbtA.

Meanwhile, a voltage VsigB is generated across the reference electrode13 and the top gate of the sensor transistor 17B in accordance with theconcentration on ions of another specific substance in the samplesolution 15. The constant voltage source 203 supplies the constantcurrent source 201B connected with the drain of the sensor transistor17B with the constant voltage Vpw with respect to the referencepotential. The array driver circuit supplies the drain of the sensortransistor 17B with a constant current IrefB. The drain current IdB ofthe sensor transistor 17B is equal to the constant current IrefB. Theconstant current source 201B is maintained at a constant potential withrespect to the reference potential. The source of the sensor transistor17B is maintained at a constant reference potential.

The array driver circuit 25 further includes an operational amplifier205B. The operational amplifier 205B is configured as a voltagefollower. The potential of a node between the constant current source201B and the drain of the sensor transistor 17B is input to thenon-inverting input (+) of the operational amplifier 205B. The output ofthe operational amplifier 205B is connected with the inverting input (−)to apply negative feedback to the operational amplifier 205B.

The array driver circuit 25 further includes a constant voltage source207B between the output of the operational amplifier 205B and the bottomgate of the sensor transistor 17B. The constant voltage source 207Bsupplies the bottom gate of the sensor transistor 17B with a potentialdifferent from the output of the operational amplifier 205B by aconstant voltage VbtB.

The operation of the circuits for driving the sensor transistors 17A and17B is the same as the operation of the driver circuit 20 described withreference to FIG. 2 and therefore, the description is omitted here. Thearray driver circuit 25 includes the same driver circuits for the othersensor transistors. Although all reference potentials in theconfiguration example in FIG. 5 are the ground potential, they can be atdifferent values as far as individual reference potentials are constant.

As set forth above, embodiments of this disclosure have been described;however, this disclosure is not limited to the foregoing embodiments.Those skilled in the art can easily modify, add, or convert each elementin the foregoing embodiments within the scope of this disclosure. A partof the configuration of one embodiment can be replaced with aconfiguration of another embodiment or a configuration of an embodimentcan be incorporated into a configuration of another embodiment.

What is claimed is:
 1. An ion sensing device comprising: a field-effecttransistor including a bottom gate and a top gate; a referenceelectrode; and a driver circuit configured to measure concentration ofions in a sample solution into which the reference electrode and the topgate are immersed, wherein the driver circuit includes: a constantcurrent source configured to supply a drain of the field-effecttransistor with a constant current; and a voltage follower configured toreceive a potential of the drain, and wherein the driver circuit isconfigured to: supply the reference electrode with a constant fixedreference potential; apply a constant voltage across an output of thevoltage follower and the bottom gate so that a voltage across the drainand a source of the field-effect transistor varies; and output an outputpotential of the voltage follower.
 2. The ion sensing device accordingto claim 1, wherein the driver circuit is configured to operate thefield-effect transistor in a saturation region.
 3. The ion sensingdevice according to claim 1, wherein electrostatic capacitance per unitarea of an insulating film for the top gate is larger than electrostaticcapacitance per unit area of an insulating film for the bottom gate. 4.The ion sensing device according to claim 1, wherein the field-effecttransistor further includes a probe film attached on the top gate, theprobe film having a characteristic that interacts with a specificsubstance in the sample solution.
 5. The ion sensing device according toclaim 1, wherein the field-effect transistor is an oxide semiconductorthin-film transistor.
 6. A method of driving a reference electrode and afield-effect transistor including a bottom gate and a top gate in orderto measure concentration of ions in a sample solution, the methodcomprising: supplying the drain of the field-effect transistor with aconstant current; inputting a potential of the drain to a voltagefollower; supplying the reference electrode with a constant fixedreference potential; applying a constant voltage across an output of thevoltage follower and the bottom gate so that a voltage across the drainand a source of the field-effect transistor varies; and outputting anoutput potential of the voltage follower, wherein the method isperformed in a state where the reference electrode and the top gate areimmersed in the sample solution.
 7. The method according to claim 6,further comprising: operating the field-effect transistor in asaturation region.
 8. The method according to claim 6, whereinelectrostatic capacitance per unit area of an insulating film for thetop gate is larger than electrostatic capacitance per unit area of aninsulating film for the bottom gate.